Traditionally, the camshaft (or, in a multiple camshaft engine, camshafts) of an internal combustion engine, which actuates the intake and/or exhaust valves, is connected to the crankshaft, which receives the force from the pistons, by a timing chain, belt or gear arrangement driving sprockets, pulleys or gears, respectively, on the ends of the shafts. The relative timing of the camshaft(s) and crankshaft in such a system is fixed, and must be chosen to be tailored to power or economy at a given engine speed or load condition. This is inherently a compromise, as an automobile engine does not, obviously, always run at the same speed or load, and a given car owner might desire either power or economy at different times. The demands of emissions control complicate matters further.
This has given rise to Variable Cam Timing (VCT) systems, where the timing of the valves relative to the crankshaft can be changed by altering the relative rotational positions of the camshaft(s) and crankshaft. One of the more successful systems for VCT involves using a device called a “phaser” to allow the camshaft sprocket, which is linked to the crankshaft by the timing chain, to shift angular position relative to the end of the camshaft. Typically, the phaser is a coaxial arrangement of an outer housing which forms the sprocket (or pulley or gear) and an inner rotor fixed to the camshaft. The angular position of the rotor and housing can be shifted by fluid pressure acting on pistons or vanes on the rotor inside cylinders or chambers formed in the housing.
The “vane phaser” setup is commonly used in VCT systems, and will be used in the examples in this disclosure, although it will be understood that the method of the invention will work with other forms of phasers known to the art. Butterfield and Smith, U.S. Pat. No. 5,172,659, “Differential Pressure Control System for Variable Camshaft Timing System”, assigned to BorgWarner Inc., shows a vane phaser system which uses the inherent torque reversals in the camshaft caused by the actuation of the valves to move the vane from one position to another. Fluid is led from one side of each vane to the opposing side through a valve. When the valve is open, the rotor is free to oscillate, the fluid passing freely from one side of the vane to the other. When the valve is closed, the fluid cannot flow, and the vane is held in position. By opening the valve while the torque reversal is acting to move the camshaft in the desired direction, then closing the valve, the camshaft is allowed to move, then held in place by the fluid on each side of the vane.
U.S. patent application No. 20030047395, entitled CONTROL SYSTEM FOR VIBRATION EMPLOYING PIEZOELECTRIC STRAIN ACTUATORS by Patton, Mark E. teaches a vibration control system for an engine or transmission cover on a motor vehicle to absorb and dissipate gear or chain-induced vibration uses a piezoelectric strain actuator with a passive resonant control system to absorb and dissipate vibration at a fixed resonance frequency of transmission and engine timing covers. Another embodiment uses an open-loop active control system, based on signals already existing on-board in the engine controller and a control map of phase, amplitude and frequency. Still another embodiment employs a hybrid system, combining open- and closed-loop control.
U.S. Pat. No. 6,561,146, entitled METHOD OF CONTROLLING RESONANCES IN INTERNAL COMBUSTION ENGINE HAVING VARIABLE CAM TIMING by Todd, et al. teaches a method of controlling resonances in timing drive systems for internal combustion engines having variable cam timing systems using cam phasers with the capability of being locked in position. Locking or unlocking the phaser changes the resonant characteristics of the timing drive system. The invention uses these changes in characteristics between locked and unlocked phasers to minimize the effects of resonance in timing drives by changing between locked and unlocked states as engine RPM passes through resonant points.
U.S. Pat. No. 5,327,859, entitled ENGINE TIMING DRIVE WITH FIXED AND VARIABLE PHASING by Pierik et al. teaches an engine timing drive for driving a camshaft and an accessory such as a balance shaft has a transmission member including a fixed phase output for driving the accessory and a variable phase output for driving the camshaft. A preferred embodiment incorporates a planetary cam phaser in a driven sprocket that also carries a fixed phase output gear as an accessory drive.
Further, there are a number of patents teaching methods or devices for dealing with resonance and related matters. These patents typically use tensioners whereby the tensioners are softened various means.
As can be appreciated, timing drives especially modern timing drives with their lower inertia and lower friction can develop resonances that can increase the load in the timing drive. This increase in load may be undesirable. Methods and apparatus are devised to reduce these high loads. The methods and apparatus include: making the tensioner softer by increasing the leakage of the tensioner and increasing the piston to bore clearance. The methods and apparatus further include: adding a torturous path vent plug; adding apressure relief; and adding a force release to the tensioner piston.
With the introduction of one or more cam phaser(s) into a timing drive, the increased inertia from the cam phaser can cause the engine speed at which the resonance occurs to change and may affect the operating speed of the engine operation. Typically, within a neighborhood of a particular engine speed, the tensional force exerted on a chain is bigger even than that at higher engine speeds. Therefore, it is desirable to use the newly added phaser for reducing resonance or undue tension of a timing drive.